The present invention relates to electroetching of a metallic surface, and more particularly to electroetching for fabrication of C4 structures for microelectronic interconnection.
C4 is an advanced microelectronic chip packaging and connection technology. "C4" stands for Controlled Collapse Chip Connection. C4 is also known as "solder bump" and "flip chip".
The basic idea of C4 is to connect chips, chip packages, or other such units by means of solder balls partially crushed between two surfaces of the units. The tiny C4 balls of electrically conductive solder are arrayed on the surface of one unit and are pressed against metal pads on another unit to make one electrical connection at each solder ball. C4 allows all the connections to be made in one step.
A major application of C4 is in joining semiconductor microchips (integrated circuits) to chip packages. Chips usually are made in repeating rectangular arrays on a mono-crystalline disc of silicon, several inches across, called a "wafer." Many chips are formed on each wafer before the chips are broken off. C4 balls are placed on terminal metals on the chips while they are still joined in a wafer. Wafers are made as large as possible so as to make the most chips at once, and the chips are made as small as possible. The best C4 fabrication system is one that makes very small, closely-spaced solder balls each precisely placed over a large area.
C4 is commercially important because it allows a very high density of electrical interconnections. C4 can be used with perimeter connection techniques, similar to tape automated bonding (TAB), but C4 connections can also be arrayed over surfaces. When an area is covered, the number of possible connections for a given size of unit is roughly squared. C4 balls are about a hundred microns in diameter and the connector density is on the order of several thousands per square inch.
C4 solder bumps must be rugged. A computer, with dozens of chips and thousands of C4 solder ball connections, can be rendered nonfunctional if only one of the C4 balls fails.
One method of fabricating the C4 solder bumps is by evaporation or vacuum deposition. In this process, terminal metals are first evaporated in a vacuum chamber and these metals are deposited on to a wafer through a metal mask. This is followed by evaporation of solder metal, which is deposited through the metal mask on top of the terminal metals. The terminal metals form the ball limiting metallurgy (BLM) and the solder metals form the solder balls.
One of the problems encountered in fabrication of C4s by evaporation is thermal mismatch due to the difference in thermal expansion of the metal mask and the wafer. This problem, combined with stringent requirements of aligning the vias in the metal mask with those on the wafer, limits the size of wafers that can be used.
An alternative C4 fabrication technique is the electrochemical fabrication of C4s. This technique requires one or more continuous conducting metal films, i.e., seed layers, for through mask electrodeposition of the solder alloy. The seed layer has a dual function: it provides a path for the electric current during electrodeposition of the solder and it becomes the ball limiting metallurgy (BLM) for the solder pads through etching.
The seed layer metallurgy and the etching processes are crucial to the electrochemical fabrication of reliable C4s. The seed layer metallurgy should be adherent to the underlying insulating layer and should have the ability to react with Sn upon reflow thereby forming an intermetallic. The etching process should yield a BLM edge profile that can sustain the stresses in the metal films and the stresses generated during intermetallic formation. During the etching process, the solder bumps remain exposed and act as masks for the underlying seed layers. The effectiveness of the etching process is judged by its ability to remove the seed layer without attacking the solder material and by its ability to provide the desired BLM pad size and BLM edge profile.
Several different seed layer metallurgies are applicable in C4 fabrication. One of the seed layer metallurgies consisting of sputter deposited titanium-tungsten (TiW)/phased chromium and copper (CrCu)/copper (Cu) layers is used. Yet another seed layer metallurgy consisting of sputter deposited chromium (Cr)/phased chromium and copper (CrCu)/copper (Cu) layers is also used. The TiW or Cr is the adhesion layer and Cu is the solderable layer that is consumed during reflow, forming an intermetallic. The phased CrCu layer prevents deadhesion.
Different dry and wet etching processes are applicable for the removal of the seed layers. Electroetching is a metal removal process where the metal layer on the wafer is made an anode in an electrochemical cell. The seed layers are removed atom by atom from the sample and the dissolved metal ions are transported into the solution. Depending on the chemistry of the metals and electrolyte salt, anode metal ions dissolved in solution either plate the cathode, fall out as precipitate, or stay in solution.
Uniformity of etching over an area is highly desirable in many electroetching operations. When fabricating C4 balls, uniformity is essential if each solder ball is to have the optimized structure. Variations in etching rate will occur, for example, to electrolyte velocity fluctuations, temperature differentials, "stray" currents, and contact resistance. Contact resistance is an ohmic-resistance problem unique to thin metal films like the seed layers of C4 fabrication. Having a small cross-sectional area, such films have appreciable resistance to the flow of electric etching currents, so voltage will be higher near the electrical contact points at the wafer's edge than at places far from a contact. As voltage drops off, the etching rate changes.
The variations resulting from these problems can cause non-uniformity of the C4 structures across an area and consequent failures, especially if the area is wide. Prior-art electroetching methods do not overcome these problems.